1. Field of the Invention
The present invention relates to a method of inverse quantization for transmission of data with large change of a physical quantity and an inverse quantization apparatus for the same.
2. Description of the Related Art
Recently, it is requested that audio signal communication is carried out in high precision. For the purpose of the high precision, the digitization of an audio signal is carried out. In this case, it is necessary in a small volume region that the audio signal is quantized using a small volume difference. On the other hand, it is sufficient in a large volume region that the audio signal is quantized using a large volume difference. As shown in FIG. 1, the volume of the audio signal is quantized in a small volume region S1 based on the smallest unit of "1", and the quantization indexes X are illustrated as "1", "2" and "3". In a large volume region S2, the volume of the audio signal is quantized based on the larger unit of "100", and the quantization indexes X are illustrated as "1100", "1200" and "1300".
When the volume of the audio signal as continuous data is properly quantized, a quantization index X as a discrete value is obtained. The quantization index X is subject to a binary conversion for the digital communication. At this time, if the quantization index X is large, the number of bits of the binary value also becomes huge.
A method using a table is known as the technique for preventing the production of the binary value with a large number of bits. FIG. 2 shows such a method. In this method, a quantization index X is obtained through quantization of continuous data. An inverse quantization value Y corresponding to the quantization index X is retrieved from the table and is decoded. To improve precision of the reproduced sound, the scale of the table becomes enormous even in the method using such a table. As a result, a huge memory region is used for the table in one chip.
The technique for preventing the use of such a huge table is known in Japanese Laid Open Patent Application (JP-A-Heisei 5-48905). When a large value is handled, it is generally known to use a non-linear function in a logarithm scale calculator. Such conversion is also carried out in case of the audio communication. As one candidate for standardization of the non-linearizaion in the future, the following equation is proposed in the Japanese Laid Open Patent ApplicatiOn (JP-A-Heisei 5-48905). EQU Y=X.sup.c
where X is a quantization index and Y is an inverse quantization value. A variable X is transmitted using the above function, and the value Y is determined from the received variable X. Even in this case, the size of a table necessary in the receiving side is equal to the width of the definition region of the quantization index X. Therefore, the table size has become very large. For example, the definition region of the quantization index X is from 0 to 8191 in MPEG1-layer3 decoder and MPEG2-AAC decoder. At this time, the size of table is 8192. In this way, there is a problem that the size of the inverse quantization table becomes very large in the above conventional examples.